1. Field of the Invention
The present invention relates to a technology for conveying a sheet discharged from an upper-level apparatus to a predetermined position for performing a post processing of the sheet.
2. Description of the Related Art
A typical sheet processing apparatus is configured such that it conveys a sheet received from an image forming apparatus, such as a copier or a printer, to an intermediate processing tray one by one; then performs post processing (for example, a stapling process) set in advance by a user through an operation screen of the image forming apparatus on a stack of sheets stacked in the intermediate processing tray; then discharges the stack of sheets out of the intermediate processing tray after the post processing is completed; and then receives a next sheet from the image forming apparatus.
In such a sheet processing apparatus, it is necessary to assure a processing time for performing a process on the stack of sheets in the intermediate processing tray. Therefore, feeding of a next sheet from the image forming apparatus is sometimes suspended until the process is finished, which leads to degradation in productivity.
In consideration of the fact that an interval between sheets fed by the image forming apparatus is usually constant, Japanese Patent Application Laid-open No. 2001-97631 discloses a sheet processing apparatus having a sheet buffering mechanism that can temporarily stacks therein sheets so that a processing time for performing post processing or sheet-stack discharge processing can be assured.
However, to arrange the sheet buffering mechanism in the sheet processing apparatus, a space for the sheet buffering mechanism needs to be assured inside the sheet processing apparatus. Therefore, the size of the sheet processing apparatus increases or necessary cost increases.
Japanese Patent No. 3980834 discloses a sheet processing apparatus that controls a sheet conveying speed such that an interval between sheets (time interval) can be increased so that an operational time necessary for performing a sorting process, a sheet feed back process, or an alignment process on sheets can be assured.
A sheet processing apparatus that controls a sheet conveying speed is also disclosed in, for example, Japanese Patent No. 3886135. Specifically, the sheet processing apparatus is configured to accelerate the sheet conveying speed from a first speed to a second speed at a first timing so that a distance between a sheet being conveyed by a conveying unit and a sheet to be conveyed next can be increased. When a sheet being conveyed is the last sheet in a preceding stack of sheets, the sheet conveying speed is accelerated from the first speed to the second speed at a second timing that is delayed from the first timing.
However, in such a sheet processing apparatus, when the sheet conveying speed is accelerated at the second timing, a distance for which the sheet is conveyed at the second speed becomes extremely short. Therefore, the sheet conveying speed needs to be rapidly accelerated and then rapidly decelerated. As a result, the sheet may be damaged such that the sheet is torn or dirtied, or performance of the conveying unit such as a stepper motor may be degraded.
As described above, when arranging a mechanism for temporarily stacking therein a sheet so that a time for performing post processing or sheet-stack discharge processing can be assured in the sheet processing apparatus, a space for the mechanism needs to be assured in the sheet processing apparatus. Therefore, the size of the sheet processing apparatus increases.
Furthermore, when the sheet processing apparatus is provided with a control unit that controls the sheet conveying speed, because the sheet conveying speed is sometimes changed rapidly, loads applied to various components including sheets increase. Therefore, the sheet may be torn or dirtied, or performance of the components such as a feed drive source may be degraded.